The use of endotherm-producing chemical systems in cold packs is known. Cold packs are used for the treatment of soreness of muscles, the treatment of injuries such as sprains, to reduce the temperature of food or beverage, and for other related applications. The treatment of injuries and sore muscles using a cold pack is generally referred to as "cold therapy."
In the case of cold therapy, for example, because the swelling associated with the injury or sore muscle begins almost immediately with the onset of the injury, or the stress inducing the soreness, treatment should begin promptly. Accordingly, it is desirable that whatever the source of cold therapy used for such treatment, the cold therapy source should be readily available, easy to use, and capable of providing cold therapy for a duration that is effective in treating the injury or sore muscle.
Cold packs may be of several general types. There are those cold packs that contain an insulating material which, upon cooling in a refrigerator or freezer, gradually warm back to ambient temperature. There are those cold packs that operate via a change of phase of the components of the cold pack. There are also those cold packs that employ chemical components that dissolve endothermically in a solvent.
Examples of cold packs that employ an insulating material are cold packs that contain a gel. Typically, these cold packs are cooled in a refrigerator or freezer. For cold therapy, once cooled, the cold pack is placed on the injured or sore area and thus provide the cold therapy. Typical gels are based on the gelation of xanthan gum, locust bean gum, gum tragacanth, guar gum, hydroxypropyl methylcellulose, absorbent polymers, and the like. Gels may also be based on a high molecular weight polyacrylic acid cross-linked with a polyalkenyl ether, also referred to as a cis-carbomer.
Other examples of cold packs that employ an insulating material exist in the art. For example, cold packs may employ an outer insulative layer. Alternately, clays or silicates may be used to form aqueous colloidal dispersions sometimes referred to as gels. These colloidal dispersions would perform a similar insulating function as do the gels described above.
Phase change materials may be converted between solid and liquid phases and utilize their latent heat of fusion to cool during such phase conversion. The latent heats of fusion are greater than the sensible heat capacities of the materials. Accordingly, the amount of energy absorbed upon melting or released upon freezing is greater than the amount of energy absorbed or released upon increasing or decreasing the temperature of the material by 10.degree. C. Water is one example of a phase change material.
Certain chemical compounds, once dissolved into a solution, result in a lowering of the temperature of the solution below ambient temperature. On dissolution, these compounds take up heat from the surrounding environment. For example, inorganic salts or soluble organic compounds known to have a positive (greater than zero) enthalpy (.DELTA..sub.sol H.degree.) of aqueous solution are used to make the reduced temperature solutions useful in cold packs. However, solvents other than water may be used so long as .DELTA..sub.sol H.degree. of the solute is greater than zero. Other ingredients may be added as well.
Any of these cold packs may be used in combination. Cold packs which employ a gel may also contain endotherm-producing compounds. Phase change materials may also be used in combination with endotherm-producing compounds.
All of these cold packs require the use of a liquid component. The cold packs described above may contain a liquid or require the addition of a liquid when the cold packs are prepared for use. For example, a gel pack may contain either a gel or a gelation agent. If the cold pack contains the gel, then a liquid, typically water, may already be incorporated into the gel. If the cold pack contains the gelation agent, then a liquid must be added so that the gel may form prior to use of the cold pack. A cold pack that functions based on the use of a phase change material has a liquid phase by definition. A cold pack based on the use of an endotherm-producing chemical is packaged dry and an activating liquid may be added at the time of use. Optionally, such a cold pack may contain an activating liquid kept in a compartment separate from the endotherm-producing chemical until the time of use.
One problem with these conventional cold packs is the short duration of the reduced temperature effect. To be useful in cold therapy, or maintaining food or beverages at a suitable temperature, the cold pack must provide a reduced temperature for a reasonable period of time.
Cold packs of the type that employ endotherm-producing chemicals have employed various methods to extend the cold duration or the "life" of the cold pack. These methods may be divided into three broad categories: (1) physical means to slow dissolution of the endotherm-producing chemical; (2) temperature means to provide a large temperature differential with respect to an ambient temperature; and (3) insulation means to slow the rate of heat absorption in an attempt to increase the time the cold pack is at a reduced temperature.
The first category employs physical means to slow the dissolution of the endotherm-producing chemical. Cold packs of this type have used coated endotherm-producing chemicals whereby the coating slows the dissolution of the chemical. Cold packs of this type have also used endotherm-producing chemicals pressed into pellets. The related prior art teaches that the pellet-form slows the dissolution of the endotherm-producing chemical and thus prolongs the life of the cold pack.
The second category employs a large temperature differential with respect to an ambient temperature. As the temperature differential increases, the time it takes the cold pack to return to an ambient temperature increases. This large temperature differential is accomplished by using two endotherm-producing chemicals whereby one of the chemicals reduces the temperature to an extremely low value and the other chemical reduces the temperature to that useful for cold therapy, for example.
The third category includes those cold packs which also employ gelling agents. These gelling agents may be included in the same container as the endotherm producing chemical. One example of a typical gelling agent is hydroxypropyl methylcellulose. When initiated, the endotherm producing chemical reduces the temperature of the cold pack and the gelling agent gels. The gel thus formed provides some level of insulation to the cold pack.
Each of the above methods for providing a cold pack and extending the cold duration has to some extent proven unreliable or cumbersome. Many conventional cold packs produce a useful reduced temperature of a relatively short duration. Therefore, the cold pack may be ineffective in providing cold therapy or maintaining a food or beverage at a proper reduced temperature. Attempts to extend the reduced temperature duration have presented problems particularly where the cold pack is used to provide cold therapy. If the means to extend the life of the cold pack is based on using a large initial temperature differential, the cold pack will most likely generate a temperature that is too cold for use in cold therapy.
It is generally understood that a cold pack will maintain a reduced temperature for increasing amounts of time as the concentration of endotherm producing chemical increases. It is also generally understood that endotherm producing chemicals are salts, such as ammonium chloride or ammonium nitrate. It is also generally true that cold packs which employ endotherm producing salts are used only one time. Once the cold pack has returned to a temperature at which it is no longer useful as a cold pack, the cold pack must be thrown away. Disposal regulations, however, may limit the amount of the endotherm producing salt which can be used in a cold pack. Therefore, the concentration of endotherm producing salt cannot be increased without limit.
What would therefore be advantageous would be a cold pack having an extended life. It would be advantageous to have a cold pack which avoids the use of potentially costly insulating means. In particular it would be advantageous to have a cold pack which provides an extended life at a useable temperature and which uses a salt concentration which was permissible in view of disposal regulations.